1. Field of the Invention
This invention relates to a thermally responsive control for a damper of the type used in the flue or vent of a burner such as a furnace to close the damper off when the furnace is not in operation and more particularly to such a damper control employing a fluid expansion chamber sensitive to vent temperature.
2. Prior Art
In a conventional furnace or boiler for heating a building, the combustion products heat a fluid transfer media such as air or water and then pass to the atmosphere through a chimney vent. When the furnace is not operating, heated air from within the building may be lost to the atmosphere by passage through the furnace air intake, through the combustion chamber, and up the vent. To prevent this heat loss and thus improve the operating efficiency of these devices, dampers are often employed in the vent to close off the chimney when the burner is not firing. One class of control systems for these dampers employs the same electrical signal which opens the fuel valve to the burner to open the damper. In the event that a control system of this type malfunctions, as by the fuel valve remaining jammed in an open position when it should be closed, the noxious combustion gases may back up into the heated building, endangering the occupants. To obviate this possibility, a class of dampers has been devised which sense the temperature in the furnace to detect furnace operation, and automatically open the damper whenever the temperature exceeds a predetermined value.
A number of these control systems have employed complex sensor systems or motor drive systems and have proven expensive and unreliable. Another class of devices is simpler and uses the motion of a bi-metal subjected to a changing temperature to control the motion of vent damper. These devices suffer from the corrosion that occurs because of the high temperatures that must be sensed as well as the corrosive atmosphere and many have proven unreliable in operation.
An alternative form of automatic thermal control system for burner dampers has heretofore been used to control the combustion air inlets to burners. These units employ the expansion of a gas subjected to burner combustion products to open an air damper. These devices are simple so as to be low in cost and reliable in operation but do not provide the relatively large motion required to move the vent damper from a full off to a full on position. For example, U.S. Pat. No. 136,291 discloses a damper supported in the air inlet passage of a furnace and controlled by an automatic apparatus including a fluid reservoir disposed in the combustion chamber and connected by a tube to a chamber formed on one side of a diaphragm. Motion of the diaphragm, resulting from a change in the pressure in the reservoir, controls the position of the damper as a function of the furnace temperature to regulate the air in-flow to the furnace. While this form of proportional damper regulation is simple and reliable in operation, its principle is not applicable to the problem of vent damper control wherein the damper must be moved between a full open and a full closed position each time the burner goes on or off.